1. Field of Invention
This invention relates to apparatus for the processing of semiconductor wafers for the transistor and integrated circuit industry and more particularly to apparatus for diffusing PN junctions into semiconductor material and still more particularly to sensing apparatus which monitors the flow of gas to a diffusion furnace.
In the manufacture of semiconductor devices by a single or double diffusion process, a semiconductor wafer is masked in appropriate areas so as to provide windows in the masking. PN junctions are created through these openings by exposing the wafer together with a batch of other wafers to a gaseous atmosphere containing a dopant ion for usually a matter of minutes although some diffusions may require hours.
Semiconductor wafer batch processing within a diffusion furnace requires a very reliable, repeatable and accurate apparatus, simple in design and adaptable to electrical binary signal, for the purpose of monitoring relatively low gas to flow through separate and individual gas supply tubes leading to the diffusion furnace. In general, the control of these gas supplies has been by visual and manual techniques.
Recent emphasis has been placed on the development of methods and apparatus capable of operating on a continuous basis and simultaneously maintaining such aspects as quality control and repeatability of operation for accurately controlling the flowing and mixing of gases to a diffusion furnace. The gas flow within the diffusion furnace produces convection effects which are highly affected by the accurate flow of new gas. Because of the extreme criticality of processing parameters necessary for the processing of silicon wafers within a diffusion furnace, substantial developmental effort has been directed to devise methods and apparatus for accurately and reproducibly monitoring the flow of new gas to the diffusion furnace.
2. Description of Prior Art
Prior art apparatus used for measuring and controlling the flow of gases to a diffusion furnace where semiconductor wafers were being processed consisted of a standard flowmeter or manometer (a graduated glass cylinder) and manual control valve.
Where the accurate mass transfer within a flowing gas was considered critical manufacturing differences from one flow meter and control valve to another rendered an intolerable condition.
Flow meter readings were interpreted differently and different techniques for adjusting gas flow to the diffusion furnace were applied; thus leading to fewer acceptably processed semiconductor wafer batches.
Prior art apparatus prove to be undesirable from the standpoint of individual variations in the apparatus used to sense gas flow and control gas flow.